Accumulators are traditionally used in hydraulic circuits to provide or absorb momentary flow of hydraulic fluid, to attenuate pressure fluctuations in the hydraulic circuit and minimize the resulting degrading of host system components. The extra capacity provided by an accumulator also enables the overall system to be designed with smaller pumps without loss of performance. An accumulator serves to provide an extra volume of hydraulic fluid at a specific point in the circuit which has been maintained at system pressure, most typically by the force of a compressed gas. In that form, the accumulator serves as an auxillary power source, pulsation damper, fluid dispenser (discharging a known volume), suction stabilizer, leakage make-up source, or thermo expansion volume compensator.
Most accumulators pressurize the hydraulic fluid therein by the use of compressed gas separated from hydraulic fluid by a permeation resistant resilient bladder. Springs and weighted structures have also been employed to pressurize fluid within an accumulator.
Fluid-filled accumulators have been employed in vehicular braking systems due largely to their relatively simple and compact structure. They can be particularly advantageously applied in anti-lock brake systems (ABS) when located near the outlet of an associated hydraulic pump which is used to generate supplemental brake fluid pressure required to actuate the brakes at the wheel cylinders through an ABS hydraulic control unit independently of the brake pedal being actuated by the operator. The accumulator assists in significantly reducing the amplitude of the brake fluid high energy pressure pulses at the outlet of the pump, thereby helping to eliminate or substantially reduce the mechanical noise hydraulically generated in the braking system and traveling through the master cylinder or its connection to the vehicle into the passenger compartment.
A conventional accumulator, as assembled, in effect has no moving parts. It basically comprises a closed chamber filled with brake fluid having an inlet from the high pressure output side of the pump and an outlet through an orifice of substantially reduced diameter. Because of the restriction of flow through the orifice, the pressure fluctuations are attenuated as a result of the compressibility of the brake fluid itself, i.e. absorbing the high energy fluid pulses and slowly releasing it through the orifice.
A conventional accumulator used for this purpose typically has significant axial length as it is required to (i.) facilitate several O-ring seals confirming the brake fluid to the accumulator and assuring its prescribed flow through the accumulator, and (ii.) assure the isolation of the incoming fluid to the accumulator reservoir and through the outlet orifice, thereby avoiding any short circuiting of the function of the accumulator. This conventional design also requires a bleed port and fitting for allowing a means of eliminating entrapped air from the brake fluid line. It also requires a compression spring for maintaining the attenuator cylinder firmly against a retaining clip, holding the cylinder in fixed position relative to the hydraulic control unit.
The use of solid elastomeric members in accumulators has been proposed to absorb shock in high pressure hydraulic systems. For example, U.S. Pat. No. 5,540,486 to Linkner, describes a hydraulic control unit for a vehicle anti-lock brake system including an attenuator made-up of a compresssibly deformable solid core member disposed within a sealed chamber. The attenuator's deformable member is configured to substantially fill a chamber but to provide a clearance relationship between the deformable member and the chamber walls to thereby expose most sides of the deformable member to brake fluid pressure.
Although apparently overcoming some of the aforementioned shortcomings of conventional accumulator structures, the accumulator described in the '486 patent has certain limitations. Specifically, by spacing the deformable member from the walls of the cavity, under pressure, the deformable member will contort in an unpredictable manner fictionally engaging and moving locally with respect to adjacent cavity surfaces, leading to unequal distribution of pressure on a deformable member which causes hysteresis due to mechanical friction between the member and the cavity walls. Furthermore, although the abstract of the '486 patent states that all sides of the deformable member are exposed to brake fluid pressure, that simply is not the case. This attenuator has, in fact, many of the same infirmities as earlier solid elastomeric accumulators inasmuch as a substantial part of the elastomeric member (that which is abutting head member 44) remains in intimate contact with the cavity wall, thereby creating pressure asymmetries imposed by the hydraulic fluid on the deformable member.